First off replace all 4 power tubes together with 4 new tubes. Then bias according to manufacturer recommendations. If the amp is old and the tubes are that tired , I’d consider replacing the small signal tubes as well. Be careful not to damage the amp with a faulty tube, or powering it up without a speaker load. As far as your question , I’d consider the new production Gold Lion KT 88’s, great tube. They will be slightly less power and slightly less bass, but would be fine. I’ve also witnessed my friend going to Gold Lion KT 77’s in his M 180 mono blocks. Rogue ships a lot of its gear with JJ small tubes, and there is a ton of improvement to be had. This is all based on the assumption that your amp is healthy and has no other issues. Regards , Mike B.
KT120 replacement options in a Cronus Magnum II
I have a "new to me" Cronus Magnum II" that came with a shot KT120. Thankfully the am still plays with half the power on the one channel. I'm having no luck finding this tube. Being made in Russia and in high demand I expect the situation to stay lean for some time. Rouge reported that the KT88’s, KT90’s, and KT77's are all compatible. I'm new to the tube amp space and I was hoping to read for a few months before going tube shopping, but I have a situation.
Has anyone "downgraded" the Magnum II to one of these other tubes? I'm guessing I should buy a pair so both channels have the same output. Is that right?
What are your thoughts for the best way forward? I spent most of my budget so a pair of KT150's is really not an option.
Agree, replace all 4 tubes and re-bias when you're changing types. Also agree the Gold Lion KT88's are great tubes. I've rolled them into my Rogue Apollo Darks and I think there's a good argument to be made they're better than the stock KT120. The GL 88's have a sweeter midrange and an overall more refined sound.
In the past I compared EH KT90 versus KT120 - the KT90 are brighter and leaner. I greatly preferred the KT120, though systems which lean a bit dark (need a lift in air and sparkle) could conceivably benefit from the KT90.
In other amps (VAC amps - not Rogue) I've compared EH KT88 versus GL KT88. Here the EH KT88 are a bit more rock 'n roll; the GL are a bit more refined and smooth. I put the Gold Lions back in, but I really like the EH 88 and keep them around for a change of flavor/pace some day. I don't think you can go wrong with any of the modern Russian KT88, and the Gold Lions are an easy general recommendation.
That sucks about the KT120 shortage - a lot of tube amp makers have been using that as a "stock" tube in their amps for a while now, and I wonder if the tooling (dating back to 2009 / 10?) is getting worn down :(
Budget is a bit of an issue. I understand it may not be ideal, but what is the downside to getting a pair of KT88s and holding the remaining working KT120 for when the next KT120 goes?
I don't think the tubes are as much tired as the previous owner never heard of biasing them. There was 1 position where the bias was way off! I'll bet that's where the now dead one sat. The amp let's me bias per tube and it was the first thing I did after an initial warm up period and how I discovered I was down 1 tube. Okay, yeah, I did not notice that 1 tube was not glowing. I was just playing the amp quietly and working across the room. My listening room is also my work from home office.
As a data point, the amp sounds absolutely amazing even with only 1 tube on the one channel. Yes, if I drive it hard the one channel starts to deteriorate, but that is not normal for me. My, also new to me, Cerwin Vega E312s are very efficient and I don't need to push the amp to more than fill my room.
I'm looking forward to rolling the other tubes, but I need to let the buying settle a bit first. In the past month I got the Cronus Magnum II, Denon DN-300Z, and E312's. My credit card is a bit warm. ;) I was hoping to upgrade my stylus to the AT-VM95SH. I have to weigh the sonic impact of 4 tubes and no stylus upgrade with 2 tubes and a stylus upgrade.
Are you running the amp with empty tube sockets ? So tubes are a wear item, they break in and then slowly erode after hundreds of hours of use. It’s never good to run a bunch of mismatched tubes. As per your question, I’d buy tubes before a new cartridge. If a tube fails it might result in catastrophic damage to the amp. I’ve had multiple tube failures in my lifetime, most are harmless, but I had one destroy the power supply and the amp had to be shipped to the builder for repairs. Try to access a tube tester and test your 120’s, then you’ll know for sure. Find a repair shop in your town and see if you can test them. You keep mentioning pairs, but you need a quad (4) . At least I’d get 4 new tubes and if you get more 120’s you can test yours and keep for spares. The amp you have is a great amp, but they have a reputation for eating tubes. Good luck, Mike.
@markcasazza do you feel confident its just the one KT120 going bad? Have you swapped R/L tubes to see if the issue follows that one tube?
If its just the one tube you might be able to find a single for now and let it play a while longer while saving back up and deciding to pick up a different set of GL KT77/KT88s later on. A nice luxury to be able to run these smaller musical tubes too in your amp.
Great questions. Here are the answers:
I have swamped the tube and the issue follows the tube. I get no current when trying to bias and the tube does not get warm has a burn spot on the its side unlike the "hot spot" of 3 other tubes.
Rogue Audio said it is okay to operate the amp with the dead tube. It is inserted. I turned it off at first until they advised it was okay.
I finally tracked down a KT120 for sale. I'm now thinking to buy 2 KT120's and get the 3 remaining working tubes tested. Set the weakest aside as a spare. Delivery on a new tubes will be 3 weeks. 🙁 Getting the tubes tested is also not trivial as the nearest shop that I know of is an hour's drive away. I will keep looking closer to home, but I live in rural Ohio.
I like the idea of a full set of KT88s sitting in a drawer. That may be a Christmas present to myself.
@markcasazza Best to follow this reply, from a known member with first hand experience on both sets of 88/120 tubes in that specific CMII amp and version you have. FYI tubes can perform and sound differently in different tube amp circuit designs and across different designer-builders. Glad to see this reply too, I run KT120s in my current mono amps myself. While my current quad of 120s and 150s are okay, and I have a spare quad of KT120s. Checking this morning - my own tube source provider reports they may not see new quad sets of these larger KTs again until late April/May ’23. So, good news you found a KT120 or two. Worth trying first, yes.
Indeed, I had the resort to eBay and there is only 1 seller shipping from China. I just ordered 2 so I have a spare. I suspect the war is causing import issues. No sign that will be over any time soon. Now I wait 3 weeks.
Avanti1960, did you roll any of the pre-amp tubes? That was my plan come next winter. I saw 1 thread indicating some success. I'll bookmark them for when my wallet recovers.
@markcasazza yep, clearly a New Sensor TungSol tube and supplier shortage again for sad reasons of war, yes, likely. Recently I checked my former sources and a few others. Some TS tube suppliers I know of are reporting the next round of TS output tube supplies might come back in stock in April/May. But, I would not count on it.
Also just for grins I’m checking with a former overseas source provider of the prior available KT120s that use to be with label by PSVANE as KT120s, saw those a few years ago but never tried them myself. Some other non-TS options would be nice.
As a backup I recently acquired up an older familiar SS amp to rotate in once in a while any time I want to give the KT120s and KT150s a rest. If that does not go well, will plan on obtaining an EL34 amp or something. Other options nice to have.
You are correct, do not mix tubes of different values in your amp. Also great advice above from the member comparing the specific tubes in the same amp. As far as “ Hot Spots “, are you referring to the Ghetter Flashing ? Watch some Utube videos that show how this happens. You will see how they use inductance to heat the Ghetter material and how it collects the impurities and deposits it on the glass envelope. When a tube fails you will sometimes get a black deposit on the glass. Also if the envelope fails , you will get a grey or white deposit on the envelope. Since you now have a tube amp, take some time to search on the internet. Videos on tube manufacturing are fascinating and reading about how tubes function is very interesting. Having a minimal knowledge of how tubes are made and function has served me well. Also there are many members here that are extremely knowledgeable on this subject. As a kid I built some Dynaco stuff, as well as some SS kits. I found it very rewarding. Good luck and happy listening. If your 120 purchase doesn’t materialize, PM me and I’ll help you out. Regards , Mike B.
If this is PcX,Tube New Tung-Sol KT120 (Super 6550/KT88) (partsconnexion.com) they sold out already. I got 2 off ebay and they work great. I have a spare now for the inevitable.
Tried to make the switch from 120’s to 88’s in my Atlas Magnum III to save some money, but it was so much better with the 120’s in my system that I just had to spend whatever it cost. Fortunately, I found a matched quad of Tung Sols for $400 and was given a single replacement to try to get the most out of the 3 I was left with.
The world needs more KT120 tubes. They’re musical and authoritative; truly fantastic and hopefully a future-proof, new classic. Good luck with your hunt.
Probably so. I’ve seen this comment when amps are not truly optimized to run KT120s or KT150 tubes properly within their optimum operating window. Just because the tube fits in the socket, does not always mean it’s going to sound its best in any amp. Worth researching more. Is the amp truly designed to run KT120/KT150s well? Ask the designer for the plate voltage, rec bias - check values of the tube spec vs. the amp capability. Compare.
Running a KT120 or KT150 tube "cool" in an amp with lower plate voltage and low bias current, well under its optimum operating window, will have more of a sterile un-engaging sound. Drop in a smaller KT77/KT88 in the same amp, running it hotter inside its desired operating range - now sounds nice. How is that, hmmm.
Now, move that same set of KT120s or KT150s to an amp with bigger transformers, i.e. running 600v at the plate, biased at 60-70ma, moving up to 70-80% plate dissipation, now the same tubes running inside their optimum operating window, no longer being run "cold" or "cool", now sounds like a nice sounding tube amp again. The BlueGlow Electronics guy has a nice YouTube video about this, running tubes "in their window". No need to run big tubes in smaller low current amps not designed to run them properly - just a waste of money, fwiw.
The CM II or CM III are not low current amps. They come with KT120s but can run KT88s per Rogue.
KT120 had been stock in Rogue amps for many years now. You can run the KT120 in these amps at a higher bias (40 - 45mA, or even 50mA if you’re bold) than KT88 (35mA). In operation, they should be fairly close with more headroom (plate dissipation) on the KT120. For both the Rogue Apollos and VAC 200iQ’s I’ve rolled tubes in, Gold Lion KT88 vs. KT120 is just a matter of preference. KT88’s are more refined, slightly sweeter midrange; KT120 have more fullness and bass presence, overall a very voluptuous and "powerful" sound -- this is why Rogue switched to them (also they suffered through many faulty batches of Electro Harmonix KT88 in the 2000s). But the KT120’s upper mids can be a little rougher and drier in comparison. I didn’t even notice that for the first few years, but over time it wore on me a bit. That’s why I switched to Gold Lion KT88 in the Apollos, which aren’t naturally sweet like the VAC amps.
KT150 is a very different tube; they were safe to run in 200iQs but sounded awful. That’s where I buy the argument that an amp needs to designed *specifically* for that tube. But KT120 vs. KT88? Nah, they’re close enough for many models with adjustable bias and auto bias schemes. Roll and have fun, if you’ve got ’em.
Currently running Mullard KT88 in my 200iQ’s, and these tubes are excellent as well -- they will definitely sound great in the Rogue amps. But the KT120 are really excellent in those VACs. The VAC’s midrange covers for the KT120. I hope production files in again.
@tubeguy76 Curious to know what the actual plate voltage is in the new CM III? Is it static, or does PV change when KT88s are installed? (if anyone here knows the design well) and has the real specs. I’ve looked for this before with Rogue, would be interesting to know and share here.
Note: I’ve seen this complaint before about KT120 or KT150s not sounding as good as KT88s in the Rouge amps starting back with CMII, and with the CMIII too. Always wondered why not.
Sure, there are many amps that can run both with lower plate voltages, yet how the circuit, transformers, design is set up is key in how the larger KT output tubes behave, performs, sounds. The circuit design, voltage, transformers - matters.
i.e. I cannot run KT77/KT88s in my Quicksilver Mono 120s with the larger transformers and higher plate voltage specifically designed to run KT150s. KT120s are okay too. The big monos will burn up KT88s, right on the ragged edge at 600v for most KT88s. It’ll work, but will burn them up prematurely. Having owned amps with EL34/KT88s, KT120s, KT150s, can share KT120s sound glorious in the QS Mono 120s, sound full, powerful, plenty of midrange, can be quite liquid when the big iron gets warmed up and saturated. Similar to VAC, in the right amp design KT120s can sound very nice. Same with Conrad Johnson too with KT120s, nice!
@mulveling I’m with ya man, only have so many spare quads of KT120s and KT150s lying around. If New Sensor does not figure out how to get them out, its only a matter of time before PSVANE and others start producing clones, Read something recently about Mike at NS/Sovtek reaching out to PSVANE regarding this. Maybe another path of getting production out to clients. Will be tracking this.
Unfortunately, I think Psvane DID try to make KT120 several years ago - and it was a complete bust due to failures and general poor reliability. Then their KT120 just quietly disappeared from the market. That doesn’t bode well for them filling in KT120 production.
They did, and yes, several years ago. Same was said about PSVANE 6SN7s, and KT88s too, and fast forward five years, some changes for the better.. New people and equipment acquired since. So far so good on my last pairs of CV181 MKII 6sN7s and helped a few friends re-tube with quads of PSVANE KT88s, no issues past two years.
And now making tubes for some high-end amp manufacturers, easy to spot, see them at my local audio dealer. Several tubes out now since. Quietly working in the background. Counted 75 different versions and levels of tubes in production now this week. They don't seem to be slowing down yet.
@decooney - that’s fair! I might give them a shot if and when their new KT120 hits market. I still kinda want to be a tube snob, but my desire for good sounding tubes (that are actually available) far eclipses that. And I have to admit the VAC-selected Shuguang / China 12AX7’s they use in their gear sound DAMN good - some of the finest!